SEISMOLOGY AND OCEAN SCIENCES
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Program Aims to Reduce Impact of Tsunamis on Pacific States
Earthquakes are not the only seismic risk that residents of the Pacific coast face. Tsunamis--sea waves produced by large-scale disturbances of the ocean floor, such as submarine earthquakes--also threaten lives and property.
by Eddie N. Bernard, NOAA Pacific Marine Environmental Laboratory, Seattle, Wash.
On July 17, 1998 over 2000 people lost their lives when a tsunami hit the coast of Papua New Guinea only a few minutes after being triggered by a strong earthquake just offshore. To lessen the impact of events like the Papua New Guinea tsunami, a state/federal partnership is underway to protect vulnerable U.S. communities along the Pacific Coast. Now in its second year, the National Tsunami Hazard Mitigation Program is continuing to prepare tsunami inundation maps, construct deep-ocean tsunami detectors, upgrade seismic networks, and develop mitigation plans for each state along the Pacific coast. Goals include providing early warning of tsunamis and educating residents about evacuation. Coastal communities in Alaska, northern California, southeastern Hawaii, Oregon, and Washington are particularly threatened by tsunamis generated by local earthquakeslike the Papua New Guinea event. A number of large, local earthquakes in the past 2000 years have generated tsunamis resulting in sudden and extensive flooding along the coastlines of Washington, Oregon, and California. The earthquake zones shown in Figure 1 subject all five Pacific states to local tsunami threat. These zones and other seismically active areas of the Pacific rim also expose these states to distant tsunami hazard.
| Figure 1 |
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Fig. 1. Tsunami hazard for the United States is defined by the earthquake zones capable of generating tsunamis in the Alaska-Aleutian Seismic Zone, the Cascadia Subduction Zone, and Hawaii. New seismic and tsunami detector stations have been established where population centers are at risk from tsunamis. The inset illustrates the workings of the tsunami detectors, which communicate from the seafloor to the surface buoy, to the NOAA satellite, and then to the warning centers. |
The five Pacific states have teamed up with three federal bodies--the National Oceanic and Atmospheric Administration (NOAA), the Federal Emergency Management Agency (FEMA), and the U.S. Geological Survey (USGS)--in the effort. Some $2.3 million in federal funds were provided each year in 1997 and 1998. The catalyst for the program was a small tsunami generated by a magnitude 7.1 thrust earthquake, the Cape Mendocino earthquake, in northern California in April 1992. The event raised concerns about a tsunami threat to the Pacific coast, and a series of workshops led to an implementation plan for the National Tsunami Hazard Mitigation Program. The plan called for the application of existin technology to reduce tsunami hazards. The program has its work cut out for it. Local tsunamis give residents only a few minutes to seek safety. Warnings are issued for distant tsunamis, but there is the need for accurate, reasonably rapid assessment of the tsunami hazard to avoid costly false alarms.
Complicating the picture, local earthquakes can generate tsunamis many miles away. Thus residents in Alaska can experience a local earthquake and tsunami while residents of Hawaii and the west coast may experience this same disaster as a distant tsunami. Similarly, west coast residents can experience a local tsunami that may also affect Alaska and Hawaii. Of the two tsunami types, local tsunamis are the more devastating. The challenge is to design a tsunami hazard mitigation program to protect life and property from both types of tsunami events.
When a large subduction zone* earthquake occurs, the first tsunami waves may reach nearby coastal communities within 10 minutes of the event. For example, a 1993 tsunami in the Sea of Japan struck the town of Aonae about 5 minutes after the earthquake. Of the 1400 people near the tsunami source and at risk of being killed, 200 died. Survivors reported feeling the strong earthquake and, well aware of the potential tsunami hazard, immediately fled to higher ground. This illustrates the critical importance of prior public awareness as an effective mitigation tool in areas potentially vulnerable to tsunamis. Communities must know in advance what areas are likely to be flooded in order to designate evacuation routes and identify safe regions in which to assemble evacuees. Planners, emergency responders, and residents must also understand that much of the community infrastructure will be disrupted by a very large, local earthquake. Thus a sustained public outreach program is needed to educate and gain the long-term support of coastal populations for the incorporation of tsunami mitigation into an all-hazard risk reduction plan. The major tsunami produced by the 1964 Alaska earthquake was generated by a broad 700-km-long zone of seafloor warping along the Alaska-Aleutian subduction zone. Tsunami waves arrived at Kodiak Island about 30 minutes after the earthquake. Secondary slump-related tsunamis produced by submarine slides in Prince William Sound arrived at Valdez only minutes after the earthquake. Over 90% of the 116 deaths from the 1964 earthquake resulted from local tsunami waves. The Cascadia subduction zone along the Pacific Northwest coast presents a geologic setting similar to the Alaska coast. The proximity of the southern portion of the Cascadia subduction zone to the coastline makes it likely that tsunami waves will arrive along the coasts of northern California and southern Oregon only a few minutes after a large earthquake. Along the more northern parts of the Cascadia subduction zone, waves might arrive as quickly as 20-40 minutes after a large earthquake. A substantial population is at risk from such a tsunami; approximately a half million people live, work, and travel through potential inundation areas along the Cascadia subduction zone. Communities on coastlines bordering subduction zones are at the greatest risk of significant local tsunamis. However, other Pacific coast communities must also consider the local tsunami hazard, since large, nonsubducting, earthquakes may trigger local submarine landslides that can generate destructive tsunamis. In fact, several poorly documented local tsunamis caused some damage to southern California communities in the 1800s; three of which produced flooding in the Santa Barbara area. A magnitude 5.2 earthquake in 1930 reportedly generated a 20-foot wave in Santa Monica. Hawaii has experienced at least six local tsunamis since the mid-1800s. In 1975, a magnitude 7.2 earthquake near the southeast coast of Hawaii produced a local tsunami with wave heights of at least 20 feet and a maximum in one area of nearly 45 feet. Two deaths and property damage of $1.5 million were attributed to this tsunami.
All of the U.S. coastline bordering the Pacific Ocean is exposed to distant tsunami dangers and has experienced major damage and loss of life from tsunamis originating near Chile (1819, 1877, 1960), Japan (1896, 1933), Russia (1923, 1952), and Alaska (1946, 1957, 1964). NOAA tsunami warning center operations have greatly reduced the loss of life from distant tsunamis. Since 1946, the tsunami warning system has provided warnings of potential tsunami danger in the Pacific basin by monitoring earthquake activity and the passage of tsunami waves at tide gauges. However, neither seismometers nor coastal tide gauges provide data that allow accurate prediction of the impact of a tsunami at a particular coastal location. Monitoring earthquakes gives a good estimate of the potential for tsunami generation based on earthquake size and location, but gives no direct information about the tsunami itself. Tide gauges in harbors provide direct measurements of the tsunami, but the tsunami signal is significantly modified by local bathymetry* and the harbor, which severely limits their use in accurately estimating tsunami impact at other locations. Partly because of these data limitations, 15 of the 20 tsunami warnings issued since 1946 were considered false alarms.
| Figure 2 |
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| Time series of October 4, 1994, tsunami as recorded by a deep ocean recorder and two coastal tide gauge stations. The lower panel illustrates the geographical locations of the deep ocean and harbor sites. |
An example of this problem can be seen in Figure 2, which shows data from the tsunami of October 4, 1994, generated in the Kuril Islands, north of Japan. As the tsunami traveled to the U.S. west coast, it passed over two deep-ocean tsunami detectors and was then measured by coastal tide gauges in Oregon. One can assume that the offshore signals at Port Orford and Newport were the same, but the two harbors responded differently. A comparison of the amplitudes of the first positive wave cycle shows that the deep water tsunami was amplified from 1.7 centimeters to 3.9 centimeters at the Newport stations, while at Port Orford the amplification was almost twice that, from 1.7 centimeters to 7.3 centimeters. This difference in amplification can be attributed to the fact that the Port Orford gauge site is exposed to the open ocean, in contrast to the protected location of the Newport gauge (Figure 2 insets). Subsequent oscillations are determined by the continuing tsunami input and modified by the local response function, including the excitation of local and regional normal modes. The differences in these later oscillations are seen to be even more dramatic; the maximum amplitude at Newport is only 8.3 centimeters, while that at Port Orford is 27.9 centimeters--more than three times larger. Also note that these oscillations persist for more than 8 hours, possibly due to the excitation of edge waves along the continental shelf.
The National Tsunami Hazard Mitigation Program is based on three interdependent components--hazard assessment, warning guidance, and mitigation. Hazard assessment, in which the nature and level of risk is established for each individual coastal community, is an essential first step in designing appropriate warning and mitigation systems. Warning guidance, which includes physical measurement, monitoring networks, forecast algorithms, and procedures for dissemination of warnings, can be designed to meet those needs identified by the local hazard assessment. Mitigation is then based on both the hazard assessment results and the nature of the warning guidance. The goal of mitigation efforts is to ensure an appropriate response to impending tsunami danger. This requires knowledge of areas that could be flooded (assessment) and recognition of the warning system's evacuation and "all-clear" communications (guidance). Without both pieces of information, the response could be inappropriate and fail to mitigate the tsunami hazard.
For some communities, data from earlier tsunamis provide a way of identifying hazardous areas. For most communities, however, little or no data exist. For these areas, tsunami inundation numerical models can provide estimates of areas subject to flooding in the event of local or distant tsunamis. Existing inundation modeling techniques are adequate for producing tsunami inundation maps for emergency preparedness. For the first year, representatives of Oregon decided that inundation maps for Newport, Gold Beach, Seaside-Gearhart, Warrington-Astoria were needed, while Washington needed maps for Gray's Harbor, Willipa Bay, and the Long Beach Peninsula. These two states decided to use two-dimensional models developed by the Oregon Graduate Institute. The Newport, Oregon, map (Figure 3) is an example of the product of this model.
| Figure 3 |
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Fig. 3. Newport, Oregon, inundation map produced by the Oregon Deartment of Geology and Mineral Industries. |
To assist the Pacific states in developing maps identifying areas of potential tsunami flooding, the Center for Tsunami Inundation Mapping Efforts (TIME) was opened in Newport, Oregon, to implement and test tsunami inundation models, apply inundation models to selected communities, and archive bathymetric and topographic data needed to run these models. Inundation maps for Alaska and California are now being prepared.
The second element of the model is an appropriate warning system, based on local hazard assessment, to alert coastal communities of imminent tsunami danger. Three types of tsunami warning systems exist--Pacific-wide, regional, and local. The Pacific-wide system can issue a warning in about 1 hour, and is therefore useful only to populations located more than about 750 kilometers from the source, the approximate distance a tsunami would travel in this time. Regional systems warn in about 10 minutes (useful 100-750 kilometers from the source); local systems warn in about 5 minutes (useful <100 kilometers from the source). Today there is one Pacific-wide system--the Pacific Tsunami Warning Center, five well-established regional systems (two in the United States and one each in Japan, Russia, and French Polynesia), and local systems in Chile and Japan. All three system types use earthquake magnitude and location as the trigger for warnings and coastal tide stations for tsunami verification and "all clear" decisions. However, not all earthquakes generate tsunamis; thus false alarms occur mostly because the warning is based on earthquake data rather than direct tsunami measurements.
The tsunami warning system for the United States must provide both local and distant tsunami warnings for coastal communities. The first year's activities for upgrading NOAA's tsunami warning system involved installing near real-time deep-ocean tsunami detection sensors and upgrading existing west coast real-time seismic networks to quickly feed earthquake information to the Pacific Tsunami Warning Center in Hawaii and the West Coast/Alaska Tsunami Warning Center in Alaska. Locations of the real-time ocean sensors and new seismometers are shown in Figure 1. A total of six real-time ocean sensors and 38 additional real-time seismometers will be installed and maintained in future years.
During the first year, research and development was initiated to convert the real-time, deep-ocean tsunami detection system developed at the Pacific Marine Environmental Laboratory (PMEL) to an operational prototype. The system, which is composed of a bottom pressure sensor, an acoustic modem, surface buoy, and Geostationary Operational Environmental Satellite (GOES) communication link (see insert in Figure 1), was made more robust by redesigning the buoy, increasing the power and range of the acoustic modem, and improving the GOES transmissions. In September 1997, the first two systems were deployed off the Washington and Alaska coasts (see Figure 1 for location) in 2900 meter and 4600 meter water depths. These systems are designed to report tsunami data in near real time by detecting a threshold (over 3 cm) of tsunami amplitude and transmitting these data from the bottom detector to a surface buoy to the Warning Centers via GOES satellite. The delay from tsunami detection to receipt at the warning center is about 3 minutes. These data will have immediate value: they will provide direct verification that a tsunami was or was not generated and, if a tsunami was generated, they will provide its deep ocean amplitude. This information will be invaluable in assessing the threat to U.S. coastlines. With more research and development, it is anticipated that these data will eventually be incorporated into improved tsunami forecast algorithms.
Also, during the first year, an upgrade of the existing seismic networks in California, Washington, Alaska, and Hawaii was initiated to provide rapid, reliable, and relevant seismic data to the tsunami warning centers. Currently, seven seismic networks, consisting of over 1000 real-time reporting seismometers, are detecting earthquakes in tsunami source regions identified in Figure 1. With appropriate modifications, these networks can provide data and seismic analysis that can improve NOAA's ability to assess the likelihood of a tsunami.
Mitigation includes all efforts taken by communities at risk to lessen the impact of tsunamis. For the distant-source tsunami threat, it is critical that local jurisdictions receive information from the tsunami warning centers as quickly as possible, understand the information contained in warning center bulletins, and implement the appropriate response plans. A FEMA survey of 14 coastal communities after the October 4, 1994, NOAA tsunami warning resulting from an earthquake in the Kuril Islands found that 30% of local emergency managers felt the information from the warning centers was unusable, and 70% thought it was not timely. For the local tsunami, waves may arrive at nearby coastal communities only minutes after the triggering event. Even though a tsunami warning may be issued, there will be insufficient time available for local officials to accurately assess the risk, make reasoned decisions regarding evacuation, disseminate warnings to the public, and carry out an orderly evacuation. It is critical that communities not only know what areas are at risk, but that the public understand indicators of an impending local tsunami and immediately move to higher ground or inland away from the coast.
In the first year, the National Tsunami Hazard Mitigation Program formed a mitigation subcommittee that has representatives from the five states. Resource centers were established in each of the states to provide information on the tsunami warning system. A complete listing of the NOAA coordinators and their state counterparts is available at http://www.pmel.noaa.gov/tsunami-hazard/wcms.html. The latest news about the program is available at http://www.pmel.noaa.gov/tsunami-hazard.
Source: Eos, June 2, 1998, p. 258.
bathymetry -- the measurement of ocean depths and the charting of the topography of the ocean floor; strike-slip--in a fault, the component of movement or slip that is parallel to the strike of the fault;
subduction zone -- a long, narrow belt in which one lithospheric plate descends beneath another, for example where the Pacific plate descends below the South American plate
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